SETE LAGOAS, Brazil -- Uruguay striker Luis Suarez remained a great World Cup unknown on Tuesday, nursing his fragile left knee during a light workout just four days before his team faces Costa Rica in their Group D opener.

The Liverpool star had keyhole surgery on his left knee on May 22, and it is unclear how much he will play at the World Cup.

"We can't say right now when he'll be in shape to play in a World Cup match," Uruguay coach Oscar Tabarez said. "Meanwhile, reality tells us that if he's not up to playing at this level, he won't play."

Suarez trained separately from his teammates, taking soft shots and making easy passes with his right leg -- seldom using the left -- as three members of the training staff looked on. As his teammates played a training match, Suarez trotted on the sidelines.

Tabarez also left the door wide open, suggesting Suarez was coming along just fine.

"His presence in the World Cup is not being written off," Tabarez said. "We are simply increasing the demands on him daily to see how he responds."

Suarez may be rested for the first match, saving him for the more difficult games against England and Italy.

Suarez put a short video up on his twitter account on Tuesday night, saying "I wanted to tell you that the knee is going well, that I feel very good, that this depends on how things go day to day, but that everything is on a good path.

"I wanted to thank you too for the support received this week and well, everything in my mind now is the World Cup."